Marine seismic exploration includes investigating and mapping the structure and character of subsurface geological formations that underlie a body of water. A single vessel could tow seismic streamer cables and one or more seismic sources through the water. As an alternative, a tow vessel may either be a source vessel, which means it only tows one or more seismic sources, or it could be a vessel that only tows streamers, in which case two or more vessels would be used. The seismic sources may include compressed air guns or other means for generating acoustic pulses in the water. Energy from these pulses propagate downwardly into the geological formations, and is reflected upwardly from the interfaces between subsurface geological formations. Reflected energy is picked up by hydrophones of the seismic streamers, and data representing such energy is recorded and processed, whereby information about the underlying geological features is provided.
Deflectors near the vessel to pull outwardly multiple sources being towed from the direct path behind the seismic tow vessel, to maintain the transverse or crossline spacing between individual sources, rely on hydrodynamic lift created by forward motion through the water to pull the streamers and/or sources outwardly and to maintain the transverse position relative to the vessel path.
In 4-D geophysical imaging, it is important that the source members being used to generate the acoustical pulses be located as closely as possible to the same location as in previous surveys over the same grid. This has been difficult to accomplish in a marine survey because the acoustical source members are typically towed behind the tow vessel in source arrays, which are subject to wave and current movement.
Besides the deployment and operation difficulties associated with towing multiple streamers and/or multiple source arrays, conventional techniques limit the ability to position source arrays and streamers in different relative positions and orientations.
Source array design is limited by the tow configuration. Each towed source array is also subject to crosscurrents, wind, waves, shallow water, and navigation obstacles that limit the coverage provided by the survey system.
Attempts to control the location of seismic sources and source arrays have included attaching distance ropes running to lateral passive deflectors and tow cables. Use of active deflecting members attached to the source tow cables in front of the source arrays, or mid-way or at the aft end of the source arrays; and use of passive lateral deflectors equipped with a winch located near the front of the source. WO2004092771 discloses the latter two options. By attaching one or more steerable deflecting members to the front, rear, or mid-section of one or more source arrays, or a winch to the front of the source that acts on a passive lateral deflector, the source array locations may be controlled. Another method and device employs a source array comprising a rigid bar mounted under a rigid or semi-rigid float member, with the seismic source members, for example air-guns, hanging below the rigid bar.
Other solutions for controlling lateral positions of towed equipment for seismic exploration at sea are disclosed in US2008/0304357A1 and U.S. Pat. No. 7,738,317.
U.S. Pat. No. 7,423,929 discloses a steering bird for a towed seismic streamer cable, comprising a cable-steering assembly that is rotatably attached to the streamer cable. The assembly includes a body to which one or more wings are mounted, and the wings are arranged to pivot about pivot axes. An orientation sensor is employed for sensing the orientation of the wings, the cable-steering device adjusts the angle of the wings in order to provide a sideward component of force.
U.S. Pat. No. 4,323,989 discloses a steerable float for a seismic source array, having a pair of rudders at a leading edge of the float, forward of a buoyancy element. The rudders are fixedly attached to a mounting bracket forming the rudder assembly. The mounting bracket is also referred to as a support member and a cross member. The mounting bracket is connected to the leading edge of the float by a tube which forms the pivotal axis of the rudder assembly. The tube accommodates a tubular member which allows pivoting of the rudder assembly about the pivotal axis. Pivoting of the rudder assembly is controlled by an actuator coupled to a rod which is attached to a bell crank secured to the tubular member. A towing eye inserted in a reinforcing tube in a full diameter part of the float, is located adjacent to the forward end of the float, behind the rudders. The towing eye is below the horizontal center axis of the float. This assists in counteracting the rotational effects of the rudders.
The previous attempts have not provided optimal control of the location of the source arrays under varying towing conditions and different operational conditions. While these techniques are improvements in the art, further improvement is desired.